In the field of insulated cable manufacture, electrical cables containing metallic conductors and communications cables containing optical fibers and metallic conductors typically are enclosed in a polyethylene jacket. Many of those cables either carry very high voltage electrical energy or are deployed in a moist environment. Both the high voltage and the moist environment raise great concern over the possibility of a defect, or flaw in the cable jacket causing a serious failure of the cable facility once it is put into service. Such a failure is expensive to repair both because of the actual cost of making a physical repair and because of lost revenue resulting from an interruption of services.
As step toward preventing unscheduled failures of installed cables providing needed services, the manufactures of electrical and optical fiber cables have been developing apparatus and methods for inspecting polyethylene cable jackets for defects, or flaws.
One such inspection system has been disclosed by B. W. Lerch et al., in the Bell System Technical Journal, Volume XLIII, Number 4, Part 1, on pages 1225-1229. That system measures the capacitance of the polyethylene jacket. The capacitance has a constant value for all of the inspected polyethylene jacket which is defect free, or flawless. Variations of the capacitance are found anywhere along the cable where a defect is included in the polyethylene jacket or where there is an absence of polyethylene.
Another inspection system is disclosed by the Electric Power Research Institute in the following series of reports: (1) "A Far-Infrared Laser Scanner for High-Voltage Cable Inspection," Oct. 1982, (2) "Far-Infrared Inspection of Cable Insulation," Apr. 1978, and (3) "Laser Detection of Voids and Contaminants in Polyethylene-Insulated Power Cable," Dec. 1979. In this system, far-infrared light is directed at the polyethylene jacketed cable. At least some of the far-infrared light, reflected from the surface of the polyethylene or from the surface of a defect within the polyethylene, is detected by an optical device. A summary description of the inspection system, described in this series of reports, is present by J. H. Ausubel et al., Ed. in "Lasers Invention to Application", National Academy Press, 1987pages 40-44.
Both of the aforementioned arrangements have problems detecting and displaying accurate, unambiguous data relating to existing defects within a polyethylene cable jacket. The inspection system apparatus and the inspection operation cost so much or are so inaccurate that there is resistance to using them on widespread basis. Thus much polyethylene jacketed high voltage cable and optical fiber cable is being manufactured with little more than a visual inspection of the polyethylene surface or some other simple inexpensive inspection.
Thus there is a need for an accurate, low cost system for completely inspecting a polyethylene jacketed cable.
The term "polyethylene" as used herein defines a clear polyetheylene polymer which is transparent or translucent to an impinging radiation. The polyethylene is typically used as insulation for cables and conductors carrying electrical current, although some other known uses may be contemplated, also. The polyethylene may include additives, such as coloring agents or fillers, which do not affect the transparent or translucent qualities of the polymer to such an extent as to render the polymer opaque to the impinging radiation.